Vegetable · Fabaceae
Soybean
Glycine max
Give this easygoing legume warm soil and sun, and it quietly fills its own pods for you.
Full sun
Every 5–7 days
59–95°F · 15–35°C
75–100 days
Overview
Soybean is a warm-season member of the bean family, and gardeners grow it two ways: as tender green edamame picked from the pod, and as the dry beans behind tofu, soy milk, and oil. What makes it so gentle on a beginner is that it feeds itself. Tiny root nodules partner with Rhizobium bacteria and draw nitrogen straight out of the air, which is why the crop asks so little of you or your soil. Mistakes here are usually easy to recover from, so there is no need to feel nervous about your first planting.
Do not expect anything fussy in its shape. Soybean grows into a tidy, upright bush of 1.5 to 3 feet (45 to 100 cm), its stems softly fuzzy and its pods swelling in little clusters up the branches. Gather the pods while they are young and bright green and supper is edamame. Leave them longer and they dry right on the plant, until the beans rattle loose and keep for cooking through winter.
There is just one rule to remember, and it is an easy one. Unlike snap beans or garden peas, mature soybeans are not for nibbling raw, because they hold natural compounds that get in the way of digestion until heat undoes them. A short cook is all it takes, and then every bean is ready for the table.
Care
The short version: soybean wants a sunny spot and warm, well-drained ground, plus about 1 inch of water a week held steady while it flowers and fills its pods. Skip the heavy feeding, because this friendly legume supplies its own nitrogen.
Light
Give soybean a good sunny spot with eight or more hours of direct light each day and it will be happy. In deeper shade the plants tend to stretch and thin out, and you will see far fewer pods for your trouble.
Water
Aim for about 1 inch of water a week, and try to keep it even rather than feast-or-famine, since the plant leans on steady moisture most while it flowers and fills its pods. Just take care not to leave the bed sitting soggy, because waterlogged roots can turn to rot.
Soil
Soybean is easy to please here: a well-drained loam loosened with plenty of organic matter is just right. It prefers a pH somewhere between 6.0 and 6.8, the mild, close-to-neutral range that most garden beds already sit in, so odds are your soil is nearly there.
Temperature
This is a warm-weather plant at heart, comfortable across a broad range of 59°F to 95°F (15°C to 35°C). Cold soil holds the seed back, so give the ground time to warm and you will get a much better start.
Humidity
Ordinary outdoor humidity, roughly 40% to 70%, is plenty, so this is nothing to fuss over. What matters more is a bit of breathing room, since fresh air moving through the plants keeps damp, crowded leaves from giving rust an easy foothold.
Feeding
You can afford to be sparing here. Being a legume, soybean makes its own nitrogen down in its root nodules, so a low-nitrogen starter covers most soil, with a little phosphorus and potassium at planting if the ground is poor. Pile on the nitrogen and you actually switch those helpful nodules off, so less really is more.
Pruning
Happily, there is nothing to prune at all. Just pull the weeds early, while the plants are still small, and go gently around the base so you do not disturb the roots and their nitrogen-fixing nodules.
Size & spacing
Grown out, each plant stands 1.5 to 3 feet (45 to 100 cm) tall and fills 12 to 24 inches (30 to 60 cm) across. Leave a comfortable gap between them so air can drift through the rows, which quietly keeps disease from taking hold.
When to plant
Soybean simply likes to be warm, and it will wait rather than sprout in cold, soggy ground, so there is no rush. Once the last frost is behind you and the soil has settled at 60°F (16°C) or above, sow the seed straight into the bed. From there you can pick edamame in late summer, or let the pods dry in place for beans to keep.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: Direct sow after the last spring frost once soil has warmed to at least 60°F (about 15-16°C); soybean germinates poorly in cold, wet soil.
- Harvest: For edamame, harvest when pods are plump and bright green and seeds fill 80-90 percent of the pod (about the R6 stage); for dry soybeans, leave on the plant until pods and seeds dry and rattle.
Hardiness
USDA hardiness zones: 3–10
Temperature: down to -40°C
Troubleshooting
Take heart, because most soybean troubles announce themselves on the leaves long before they get serious, whether that is a chewing beetle, a sticky aphid, or a rusty spot. A quick look now and then is usually all it takes to catch things while they are still small and easy to fix. Find your symptom below, and you will know just when a gentle nudge helps.
Rust-colored bumps beneath the leaves and early leaf fall
Tan to reddish-brown bumps on the underside of the leaves, followed by yellowing and dropping, are the signs of soybean rust. Where the climate favors it, the disease can spread quickly, so if rust is known in your area it is worth confirming through your local extension diagnostic service. Caught early, a labeled fungicide together with tidying away infected debris at season end keeps it from getting ahead of you.
Fix:
- Apply a labeled fungicide preventively or at first detection in regions where rust occurs
- Remove and destroy severely infected debris at season end
Ragged holes and lacy, chewed leaves
Two beetles are behind most of this. Bean leaf beetles wear a reddish to tan coat flecked with black spots, while Japanese beetles shine a metallic green-bronze. In a small planting the calm fix is to pick them off into a jar of soapy water, and only reach for a labeled insecticide if the chewing passes about 20 to 30 percent of the leaves during leafy growth, or 10 to 15 percent once the pods are filling.
Fix:
- Hand-pick beetles into soapy water in small plantings
- Use a labeled insecticide only if defoliation exceeds roughly 20-30 percent during vegetative stages or 10-15 percent during pod fill
Sticky leaves and knots of tiny green insects
Those little knots on the new growth and leaf undersides are soybean aphids, and the sticky honeydew they drip tends to invite ants and a dusting of black sooty mold. The comforting news is that small numbers usually sort themselves out once lady beetles arrive. Hold off on any spray until counts push toward 250 aphids a plant and keep climbing.
Fix:
- Tolerate low populations, which natural enemies often control
- Treat only when populations reach economic thresholds (about 250 aphids per plant with rising counts)
Blossoms and young pods falling before they set seed
When the weather turns hot and the soil dries out during flowering, soybean will let its blossoms and tiny pods go rather than carry them on. This is heat and drought stress rather than any disease, so do not worry that something has gone wrong. Keep the soil evenly moist right through flowering and pod fill, and a layer of mulch helps hold both the temperature and the moisture steady for you.
Fix:
- Maintain even soil moisture through flowering and pod fill
- Mulch to moderate soil temperature and conserve water
Toxicity
You can relax on this one. Soybean is a food crop through and through, not a poison, so a curious cat, dog, horse, or grazing animal is in no danger near it, and soy even turns up in plenty of pet feeds. The single thing to mind belongs in the kitchen, since mature beans want a good cooking before anyone tucks in.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
A nibble of soybean leaf holds no danger for a curious cat, since the ASPCA does not list this plant as toxic to them.
Dogs None
Dogs come to no harm from soybean, and soy is common enough that it appears in some commercial pet foods.
Horses Safe
Grazing on soybean foliage causes a horse no trouble, and soybean meal is a familiar protein in many feed rations.
Rabbits Safe
Rabbits take the leaves in stride, though the green pods and beans are best offered in moderation as part of a varied diet.
Livestock Safe
Cattle, goats, and sheep take to soybean readily, and growers raise the crop widely for forage and feed.
Birds Safe
Backyard hens and other birds can eat soybean, a common part of many poultry rations, though raw mature beans are better cooked first.
People Safe
Soybeans are a staple food and safe to eat once cooked. Because raw mature beans carry trypsin inhibitors, boil or steam edamame and dry beans before they reach the plate.
This is a food-preparation note, not a poisoning hazard: raw mature soybeans contain trypsin inhibitors and lectins and should be thoroughly cooked before eating. Edamame is boiled or steamed before consumption.
Varieties
Most gardeners grow soybean for edamame, and a few dependable names come up time and again. Any of the ones below make a friendly, reliable starting point.
Midori Giant
A well-loved, large-seeded edamame that rewards you with big, sweet beans and reliably steady yields.
Be Sweet
An early edamame prized for its sweet taste and its willingness to ripen even where the season runs short.
Tohya
A neat, compact edamame that comes in fairly early and sets its pods generously.
Envy
A short-season edamame that northern gardeners count on when they want an early pick.
Companions
Soybean pairs well with corn, which feeds on the nitrogen the legume leaves behind, but it sulks next to onions and garlic, which hold back bean growth.
Plant near
corn: Corn is a traditional non-competitive companion that benefits from the nitrogen fixed by the legume.
How to propagate
There is really only one way to start soybean, and happily it is the easy way: sow the seed right where the plants will grow.
SeedEasy
Direct sow in late spring after the last frost once soil reaches at least 60°F (about 15-16°C). · Seedlings typically emerge in 7-14 days under warm soil conditions.
Soybean is raised straight from seed, and it is a friendly place to begin. Wait until late spring, once the last frost has passed and the soil has reached at least 60°F (16°C), then tuck each seed about 1 to 1.5 inches deep. In warm ground the seedlings pop up in 7 to 14 days, and if soybeans have never grown in your plot before, coating the seed with Rhizobium gives those nitrogen-fixing nodules a friendly head start.
Myths
Soybeans are fine to eat raw from the pod, just like snacking on garden peas.
Not quite, and it is an easy mix-up to make. Raw mature soybeans carry trypsin inhibitors and lectins, so they really do need cooking first. A simple boil or steam is how edamame is prepared, and that quick step leaves it both safe and easy to digest.
Soybeans need a heavy dose of nitrogen to crop well, the way many vegetables do.
Happily, no. Soybean teams up with Rhizobium bacteria in its roots and makes most of the nitrogen it needs on its own. Extra nitrogen is money down the drain, and too much of it can even switch off the very nodules doing the work.